The invention relates to a boosted braking device for a motor vehicle, of the type which comprise a master cylinder controlling the pressure in at least one braking circuit, a primary piston mounted to slide in the master cylinder to create a pressure variation therein, this primary piston being subjected to an actuating force made up of an input force exerted by a manual control member and a boost force exerted by a booster which is coupled to the manual control member.
The booster may be pneumatic and comprise a rigid casing divided into two pneumatic chambers by a moving partition which can be subjected to a difference in pressure between the chambers under the action of a valve operated by the manual control member.
A braking device of this type is known, for example from EP-B-0 662 894.
The conditions under which braking is afforded may differ. A first scenario corresponds to ordinary conditions when an obstacle is seen from afar and braking is exerted relatively gently; this braking is known as xe2x80x9cslow brakingxe2x80x9d. Another scenario is that of sharp braking or xe2x80x9cemergency brakingxe2x80x9d, for example when an obstacle suddenly appears in front of the driver who has to bring his vehicle to rest as quickly as possible.
In order to satisfy these various braking conditions, a boosted braking device, of the type defined hereinabove, has been supplemented by an emergency assist valve capable of initiating the intervention of at least two boost ratios corresponding respectively to slow braking and to emergency braking; the boost ratio of the slow braking is lower and the reaction opposing the advancement of the manual control member is greater. In the case of emergency braking, the boost ratio is higher and the reaction against the manual control member is lower, which means that the river can brake for longer and more heavily.
By convention, the term xe2x80x9cfrontxe2x80x9d will be used in the remainder of the text to mean a direction directed from the control member toward the master cylinder, and the term xe2x80x9crearxe2x80x9d or xe2x80x9cbackxe2x80x9d will be used to denote the opposite direction.
The emergency assist valve comprises a reaction piston which slides in sealed manner at the rear of a bore of the primary piston, the front part of this bore communicating with the interior volume of the master cylinder, the reaction piston having an axial passage through which there passes a ratio control pushed by a plunger itself driven by the manual control member, the ratio control at its front end comprising a head situated in a chamber lying between the reaction piston and a bushing arranged further forward in the bore of the primary piston, this bushing having a bore in which a rapid piston of smaller cross section than the reaction piston can slide, which rapid piston also has a passage open at each end, the rear end of the rapid piston and the head of the ratio control forming a valve capable of closing the inlet to the passage in the rapid piston against the action of a spring which tends to open the valve.
The slow boost ratio calls on the intervention of the reaction piston, of larger section; emergency braking calls on the intervention of the rapid piston, of smaller section.
It has become apparent that such a braking device, with an emergency assist valve, while proving entirely satisfactory from the operational and braking force points of view, gives rise to noise when the manual control member is released.
An object of the invention, above all, is to provide a braking device of the type defined hereinabove, equipped with an emergency assist valve, in which the aforementioned disadvantage no longer occurs or, at the very least, occurs only to a lesser extent.
It is furthermore desirable for the proposed solution to remain simple and economical.
In order to solve the problem, the inventors had to find the source of the noise in question. The inventors were able to determine that this noise was produced when the manual control member was released after slow braking beyond the maximum boost force that the booster can provide, that is to say beyond the xe2x80x9csaturationxe2x80x9d point, and that it was due to impact of the reaction piston against the plunger.
In order to avoid such noise, according to the invention, a boosted braking device for a motor vehicle, equipped with an emergency assist valve, of the type defined hereinabove, is characterized in that it comprises a means for preventing a sharp retreat of the reaction piston when the manual control member is released after slow braking.
The impact of the reaction piston against the plunger is thus reduced or even eliminated, as is the accompanying noise. In addition, the parts are less heavily stressed and their life is increased.
As a preference, the means for preventing a sharp retreat of the reaction piston is capable of limiting an increase in pressure in the chamber lying between the reaction piston and the bushing when the manual control member is released.
In particular, the means for preventing the sharp retreat of the reaction piston is capable of keeping the valve closed while the manual control member is being released, and of preventing a rise in pressure in the said chamber.
The means capable of keeping the valve closed may comprise a hydraulic circuit for applying the pressure obtaining in the master cylinder to a rear part of the ratio control so as to exert a force keeping the valve closed. The cross section of the rear part of the ratio control on which the pressure is exerted is greater than the cross section for closure of the valve.
Advantageously, the ratio control is made of two separate elements, a first element located forward comprising the head of the ratio control and, to the rear of the head, a part of smaller cross section mounted to slide in sealed manner in a housing in the reaction piston, this first element having a duct passing through it along its entire length, and a second element, situated to the rear of the first, of solid cross section, also mounted to slide in sealed manner in a bore in the reaction piston, this second element closing the housing in which the first element is mounted and transmitting loads between the first element and the plunger. The second element may be held in the housing of the first element by an elastic snap ring anchored in a groove in the second element.
The head of the ratio control provided at the front end of the first element may have a convex frustoconical shape projecting forward, and be fitted with an O-ring determining a diameter of the sealing zone which is smaller than the outside diameter of the rear part of this first element; the facing end of the rapid piston has a frustoconical surface which is concave toward the rear and against which the head of the first element comes to bear.
The bushing may be axially immobilized in the bore.